This invention relates to an apparatus for controlling rear wheel braking force of a vehicle to control allocation of front wheel braking force and rear wheel braking force.
When a brake pedal is pressed down, a braking hydraulic pressure (hereinafter called a "master cylinder pressure") generated in a master cylinder is transmitted to wheel cylinders of four wheel to generate braking forces on the individual wheels.
Since, when the amount of pressing down the brake pedal is increased, the braking forces generated on the individual wheels are increased, deceleration of the vehicle increases. When the deceleration of the vehicle increases, a rear wheel load decreased, resulting in a decrease in surface gripping of rear wheels. In such a braking condition that braking (high-G braking) is generated to increase the deceleration of the vehicle, there is a problem in that, when the master cylinder pressure is distributed and transmitted almost equally to the front and wheel cylinders, the rear wheels tend to lock earlier resulting in deteriorated braking stability of the vehicle.
Therefore, a proportioning valve (PCV) has been incorporated in a brake system, which functions to transmit the master cylinder pressure, as is, to the wheel cylinder of the rear wheels when the braking force is small, and reduce increasing rate of hydraulic pressure transmitted to the wheel cylinder of the rear wheels when the master cylinder pressure exceeds a preset value, thereby preventing early locking of the rear wheels.
Furthermore, there is proposed a rear wheel braking force control apparatus in which a bypass valve for bypassing the proportioning valve, the bypass valve is opened until the above preset pressure is reached, when the preset pressure is exceeded, the bypass valve is closed to function the proportioning valve, and a split point pressure is variably set to adequately control braking force allocation to the rear wheels (Japanese Patent Application 3-315660/1991).
As a prior art example which discloses a concept similar to the above point of view, in an apparatus which is disclosed in Japanese Patent Laid-open Publication 60-47750 (DE 3329706, FR 3329706), the action of the proportioning valve is normally made effective but, when an antiskid brake system (ABS) operates, the action of the proportioning valve is invalidated by an electromagnetic valve to use the rear wheel braking force up to a locking limit.
Furthermore, Japanese Patent Laid-open Publication 1-257652 (DE 3742173, FR 2624462, GB 2213543), Japanese Patent Laid-open Publication 3-208760 (DE 4029332, GB 2238092, FR 2654401) describe an apparatus in which the action of the proportioning valve is normally invalidated by an electromagnetic valve to increase braking force allocation to the rear wheels for reducing burden to a front wheel braking apparatus, but only when the ABS system malfunctions, the electromagnetic valve is operated to effect the action of the proportioning valve, thereby ensuring safety.
However, in the former prior art example, since the braking force allocation to the rear wheels is controlled by the proportioning valve before the ABS system operates, the braking force allocation tends to be excessively lowered on a road surface with a high rear wheel locking limit, resulting in an increase in burden to the front wheel brakes. When the ABS system operates, since braking force allocation to the rear wheels is increased, a rapid locking behavior of the rear wheels occurs, and hydraulic pressure control of the ABS system tends to become unstable, leading to a deterioration of control accuracy.
In the latter prior art example, despite the braking force allocation to the rear wheels is enhanced to reduce burden to the front wheel brakes, the braking force allocation to the rear wheels is maintained at a high level also when the ABS system operates, and the control accuracy of the ABS system tends to be deteriorated as in the former prior art example. Moreover, the proportioning valve can function merely when the ABS system malfunctions, and the action of the proportioning valve cannot be effectively utilized.
Therefore, it is a primary object of the present invention to appropriately control the braking force allocation to the rear wheels in cooperation with the ABS system.